How do I eliminate “checking” or “alligatoring” (the fracturing the board surface) at the decurl station?
This problem usually occurs on heavier board grades, when the Operator attempt to overcome the curl set created in the web near the core. There are four factors to consider in avoiding “checking”:
1. Core Size – If possible insure that the maximum core diameter is used. Typically board grades are wrapped on 12″ cores. The larger the core diameter the less curl set is introduced, and as a result, less mechanical decurling is required.
2. Brake Pressure – Maintain sufficient brake pressure to insure web tension to the end of the roll. For a decurl unit to work properly the web must remain taut between roll stand and cutter.
3. Penetration of the Breaker Bar or Roll – The greater the penetration into the web path of the decurl bar, the more wrap around the breaker bar and the increased likelihood of “checking”.
4. Diameter of the Breaker Roll – The smaller the diameter of the breaker roll, the sharper the angle of web wrap around the roll as it penetrates into the web path. Roll diameters of 2″ – 2¾” (50 – 70 mm) are effective on 0.024″ thick (400 gsm) board. Smaller diameters may result in “checking”.
5. In most cases, “checking” can be eliminated by reducing the penetration of the breaker and maintaining the web tension as the roll diameter decrease.
